It is well known that CT scanners irradiate an examination object with x-rays, and the attenuation of the x-rays along their path from the radiation source (x-ray source) to the detector system (x-ray detector) is detected. The attenuation is caused by the irradiated materials in the beam path, and hence the attenuation can be understood to be the line integral over the attenuation coefficient of all volume elements (voxels) along the beam path. The projection data detected cannot be interpreted directly; that is to say they do not provide an image of the irradiated layer of the examination object. Only the use of reconstruction methods affords the possibility of back-calculating the attenuation coefficient μ of the individual voxels from the projected attenuation data and hence generating an image of the distribution of the attenuation coefficients. This affords a much more sensitive examination of the examination object than by only examining projection images.
The so-called CT number is generally used for the purposes of displaying the attenuation distribution rather than using the attenuation coefficient μ itself; the CT value is a value normalized with respect to the attenuation coefficient of water. The CT number can be calculated from an up to date attenuation coefficient μ, determined by the measurement, by way of the following equation:
      C    =          1000      ×                                    μ            -                          μ                                                H                  2                                ⁢                O                                                          μ                                          H                2                            ⁢              O                                      ⁡                  [                      H            ⁢                                                  ⁢            U                    ]                      ,where the CT number C is in Hounsfield units [HU]. Water has a value CH20=0 HU, and air has a value of CL=−1000 HU. Since both representations can be transformed into one another, or are equivalent, the chosen general terms attenuation value or attenuation coefficient are used for both the attenuation coefficient μ and the CT value.
Modern x-ray computed tomography scanners (CT scanners) are used for recording, evaluating, and displaying the three-dimensional attenuation distribution. A CT scanner typically comprises a radiation source which directs a collimated, pyramid-shaped or fan-shaped beam from a focus through the examination object, e.g. a patient, and onto a detector system made up from a number of detector elements. Depending on the design of the CT scanner, the radiation source and the detector system are attached to, for example, a gantry or a C-arm which can be rotated through an angle α around a system axis (z-axis). Furthermore, provision is made for a bearing device for the examination object which can be moved or displaced along the system axis (z-axis). During the recording, each detector element of the detector system hit by the radiation produces a signal which represents a measure of the overall transparency of the examination object to the radiation emitted by the radiation source on its way to the detector system, or it represents the corresponding radiation attenuation. The set of output signals of the detector elements of the detector system obtained for a particular position of the radiation source is referred to as a projection. The position from which the beam is transmitted through the examination object is continually changed as a result of the rotation of the gantry/C-arm. A so-called scan comprises a multiplicity of projections obtained at different positions of the gantry/C-arm and/or at different positions of the bearing device. A distinction is made between sequential scanning methods (axial scan operation) and helical scanning methods.
As described above, a two-dimensional slice image of a layer of the examination object is reconstructed on the basis of a data record generated during a scan. The quantity and quality of the measurement data detected during a scan depends on the type of detector system used. It is possible to simultaneously record a number of layers using a detector system comprising an array of a number of rows and columns of detector elements. Nowadays, detector systems having 256 or more rows are known.
There are problems reconstructing the projection data if the examination object moves during the scan described above; that is to say if it moves during the detection of the projection data. Scanning a beating heart is particularly affected by this problem. Distracting movement artifacts occur in the subsequent reconstruction of the obtained projection data as a result of movements of the examination object or a portion thereof during the scan.
So as to minimize such movement artifacts, CT scanners, such as those disclosed in U.S. Pat. No. 4,991,190 and DE 29 51-222 A1, are known which comprise two scanning systems, with a scanning system in each case comprising a radiation source or a focus, and a detector system. The advantages of such CT scanners, disclosed in those documents, compared to a CT scanner comprising only one scanning system lie in the possibility of examining an examination object with an increased scanning speed or with an increased scan resolution.
High scanning speeds are particularly advantageous if movement artifacts in the reconstructed image are intended to be minimized. A high scanning speed ensures that all projections from different rotation angle positions α, used to reconstruct an image, as far as possible detect the same movement state of an object, for example the same cardiac phase of heart beats. In known CT scanners, the two scanning systems are arranged around a common rotational axis and are arranged offset to one another by an angle of 90 degrees in the direction of rotation so that the scanning speed can be doubled when suitable reconstruction methods are used.
However, CT scanners with a plurality of scanning systems can also be used to generate images with a higher resolution. For this purpose, the scanning systems are arranged around the common rotational axis so that, in the case of an identical projection direction, the projections of the two scanning systems are offset with respect to one another by a distance which is less than one detector element. A higher resolution image can be calculated by evaluating the projections detected by the scanning systems at successive times from the respective projection directions. A higher resolution is advantageous for examining blood vessels, for example, where small examination volumes have to be scanned.
The projections for reconstructing an image generated by both scanning systems are processed together in both the increased scanning speed mode and the increased scan resolution mode. The data is processed knowing the system angle which, is formed in the azimuthal direction between the scanning systems arranged around a common rotational axis.